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Metalworking (rec.crafts.metalworking) Discuss various aspects of working with metal, such as machining, welding, metal joining, screwing, casting, hardening/tempering, blacksmithing/forging, spinning and hammer work, sheet metal work. |
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Variac Help. Trying to find the right one.
I'm trying to rebuild a thermal evaporator. We need a power supply to
heat the boats (only precious metals and aluminum). I've seen very high prices for power supplies at some vendors, and I read that a VARIAC can be a good, and cheap choice. Now, what is a variac? Can it be used to evaporate gold, silver, aluminum? Can I have a variac to control 3 boats? Not at the same time but one after the other? Any help is appreciated. A confused undergrad |
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Variac Help. Trying to find the right one.
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#3
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Variac Help. Trying to find the right one.
In article ,
Andres wrote: I'm trying to rebuild a thermal evaporator. We need a power supply to heat the boats (only precious metals and aluminum). I've seen very high prices for power supplies at some vendors, and I read that a VARIAC can be a good, and cheap choice. Now, what is a variac? It is one brand of a variable autotransformer. And since it is a brand, it is spelled "Variac" -- not all upper case or all lower case. The maker -- General Radio -- I believe to be out of business now, but I could be wrong. Another maker is Superior Electric (and they call their model a "Powerstat" instead). A variable autotransformer can produce output from 0V to either full line voltage or somewhere above full -- e.g. a 120V line with 140V full scale on a tapped Variac. I have even seen center-tapped ones which could take a 120VAC input and produce 240V output (at reduced current, of course). Can it be used to evaporate gold, silver, aluminum? Not by itself. Even for the very big ones, the voltage is too high, and the current is too low -- and boats tend to be very low resistance devices, so they call for low voltage and high current. However, it can be used to adjust the voltage into a step-down transformer whose secondary is designed for the desired current, and can go to perhaps 50% over the needed voltage for the boats. Can I have a variac to control 3 boats? Not at the same time but one after the other? Yes -- with some way to switch the very high current selectively into the boat of your desire. For infrequent changes, you could unbolt the wires from the feedthrough terminals for one boat, and connect them to another boat. For more frequent changes, I would suggest high-current knife switches. The ones which I have seen used had blades of about 1/4" thickness and perhaps 1-1/4" width. They will last longer if you turn the Variac down to 0V output before switching. Any help is appreciated. A confused undergrad You have what I know about the subject -- from working with Variacs and other variable autotransformers, and observing various sized vacuum evaporation systems in use where I used to work. Good Luck, DoN. -- Email: | Voice (all times): (703) 938-4564 (too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html --- Black Holes are where God is dividing by zero --- |
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Variac Help. Trying to find the right one.
perhaps if you offered some details on power supply specs, I could help -
there are plenty of sources of high current power supplies, but without some clue as to what you actually need, it is hard to help. Do you need AC or DC, do you need a specific voltage? do you need regulation of any kind? since I'm not familiar with this specific field, I presume a "boat" is not an aquatic device for carrying passangers but some kind of smaller thing with metal in it -- are you heating it with current directly, or via a resistance element? do you have a budget? Maybe someone already knows all the answers for you, but I'd be happy to help if you can clarify the question. "Andres" wrote in message om... I'm trying to rebuild a thermal evaporator. We need a power supply to heat the boats (only precious metals and aluminum). I've seen very high prices for power supplies at some vendors, and I read that a VARIAC can be a good, and cheap choice. Now, what is a variac? Can it be used to evaporate gold, silver, aluminum? Can I have a variac to control 3 boats? Not at the same time but one after the other? Any help is appreciated. A confused undergrad |
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Variac Help. Trying to find the right one.
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Variac Help. Trying to find the right one.
I use one to cut foam. I use a thin wire then appy some current until it is
glowing then you just feed the foam , like a band saw. I have only used it a few times but it works very nice. It also "seals" the edge of the foam as it melts it. |
#8
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Variac Help. Trying to find the right one.
"Wayne" makowicki wrote in message
... I use one to cut foam. I use a thin wire then appy some current until it is glowing then you just feed the foam , like a band saw. I have only used it a few times but it works very nice. It also "seals" the edge of the foam as it melts it. Watch out for fumes. Styrene fumes are bad. Polyurethane fumes are seriously toxic. Don't use a hot wire with polyurethane foam. Ed Huntress |
#9
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Variac Help. Trying to find the right one.
In article , Andres says...
I'm trying to rebuild a thermal evaporator. We need a power supply to heat the boats (only precious metals and aluminum). You need to have a high current transformer to do what you want to do. A very small alumina coated tungsten boat will take about 50 amps or so to get any decent rate. So the typical routine for an inexpensive thermal evaporator is to use a transformer that has a 120 volt primary and a 100 amp or so secondary. The transformer's primary is then run off a variac to control the boat current. Because the boat's resistance is a strong function of temperature you need to ride the variac to keep the rate more or less constant. The easiest way to switch between sources is to ground one end of each, and ground one end of the transformer secondary. Then use a large aligator clip on the other transformer lead to connect to the other lead of each boat in turn. Companies like Denton used to use us huge mercury switches to switch between two sources. You can net a bunch of parts if you can find an old Veeco or Denton system. I myself has a tiny lab evaporator made from a scrap denton that has been retrofitted with a balzers turbopump. But it still has the original variac, transformer, bell jar, and baseplate. Jim ================================================== please reply to: JRR(zero) at yktvmv (dot) vnet (dot) ibm (dot) com ================================================== |
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Variac Help. Trying to find the right one.
if all you need is 50 to 100 amps, and IF the voltage is modest, look for an
old computer power supply (not a desktop computer) - it's not hard to find a 300 amp 5V supply surplus for $15 or so "jim rozen" wrote in message ... In article , Andres says... I'm trying to rebuild a thermal evaporator. We need a power supply to heat the boats (only precious metals and aluminum). You need to have a high current transformer to do what you want to do. A very small alumina coated tungsten boat will take about 50 amps or so to get any decent rate. So the typical routine for an inexpensive thermal evaporator is to use a transformer that has a 120 volt primary and a 100 amp or so secondary. The transformer's primary is then run off a variac to control the boat current. Because the boat's resistance is a strong function of temperature you need to ride the variac to keep the rate more or less constant. The easiest way to switch between sources is to ground one end of each, and ground one end of the transformer secondary. Then use a large aligator clip on the other transformer lead to connect to the other lead of each boat in turn. Companies like Denton used to use us huge mercury switches to switch between two sources. You can net a bunch of parts if you can find an old Veeco or Denton system. I myself has a tiny lab evaporator made from a scrap denton that has been retrofitted with a balzers turbopump. But it still has the original variac, transformer, bell jar, and baseplate. Jim ================================================== please reply to: JRR(zero) at yktvmv (dot) vnet (dot) ibm (dot) com ================================================== |
#11
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Variac Help. Trying to find the right one.
In article m, william_b_noble
says... if all you need is 50 to 100 amps, and IF the voltage is modest, look for an old computer power supply (not a desktop computer) - it's not hard to find a 300 amp 5V supply surplus for $15 or so The problem with this approach is the variable resistance of the boat as it comes up to temperature. Because a computer supply is probably a regulated switching type supply, it will be tough to ramp the temperature of the thermal source in a smooth fashion. There are really only two ways to do this, manually with a variac on the primary of a high current transformer, or using a true regulated current source supply that has a variable output. It can also be done with a light dimmer on the primary of a transformer. ================================================== please reply to: JRR(zero) at yktvmv (dot) vnet (dot) ibm (dot) com ================================================== |
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